1. Field of the Invention
The present invention relates to a surface acoustic wave filter which defines, for example, a band pass filter. More specifically, the present invention relates to a structure of a surface acoustic wave filter (SAW filter), in which a plurality of one-port surface acoustic wave resonators (SAW resonators) are connected so as to define a ladder circuit, and a method of manufacturing thereof.
2. Description of the Related Art
Conventionally, a SAW filter has been widely used as a band pass filter. For example, in Japanese Examined Patent Publication No. 56-19765, there is provided a SAW filter having an arrangement such that a plurality of one-port SAW resonators constitute a ladder circuit.
Referring to FIGS. 13 and 14, a description of the above-mentioned SAW filter having a ladder circuit structure will be provided. In the SAW filter of FIGS. 13 and 14, a series arm for connecting an input end and an output end, and a parallel arm for connecting the series arm and a reference potential are provided. A one-port SAW resonator S1 defining a series-arm resonator is connected to the series arm, and a one-port SAW resonator P1 defining a parallel-arm resonator is connected to the parallel arm. In FIG. 13, only one series-arm resonator and one parallel-arm resonator are shown. However, the number of series-arm resonators and parallel-arm resonators included in the filter is determined by the desired filter characteristics.
Referring to FIG. 14, the conventional one-port SAW resonator has an electrode structure such that an interdigital transducer (IDT) 51 has a reflector 52 on a first side thereof and a reflector 53 on a second side thereof, all arranged on a piezoelectric substrate (not shown).
The IDT 51 has a pair of bus bars 54 and 55 which extend along a direction in which a surface acoustic wave propagates. The bus bar 54 is connected to one end of each of a plurality of electrode fingers 56. The electrode fingers 56 extend in a direction that is perpendicular to the direction in which a surface acoustic wave propagates, in other words, in a direction towards the bus bar 55 on the opposite side of the bus bar 54. Similarly, the bus bar 55 is connected to one end of each of a plurality of electrode fingers 57. The plurality of electrode fingers 57 extend towards the bus bar 54. The electrode fingers 56 and 57 are arranged to be interdigitated with each other.
A plurality of the above one-port SAW resonators are arranged to constitute the ladder circuit as shown in FIG. 13, so as to define a SAW filter. FIG. 15 shows the attenuation-frequency characteristics of the SAW filter.
Since the SAW filter having the ladder circuit structure provides small insertion loss and has a wide pass band, SAW filters have been widely used as band pass filters in cellular phones or other similar devices.
Note that in Japanese Unexamined Patent Publication No. 6-232682, there is provided a one-port SAW resonator where a LiTaO.sub.3 substrate is used as a piezoelectric substrate. In this Japanese Unexamined Patent Publication, it is disclosed that if the ratio between the electrode finger cross length (aperture) of an IDT and the gap width between one bus bar and the top end of the electrode finger connected to the other bus bar is set at greater values, the effect of a ripple that occurs between the resonant frequency and the anti-resonant frequency can be suppressed.
Although the conventional SAW filter having a ladder circuit structure with a plurality of one-port SAW resonators (as disclosed in Japanese Examined Patent Publication No. 56-19765) has small insertion loss and a wide pass band, the flatness of the filter characteristics within the pass band is inadequate. More specifically, the insertion loss is less at the center of the pass band than at the shoulder ends of the pass band.
Further, in the above-mentioned SAW filter, due to the LiTaO.sub.3 substrate, the ripple that occurs between the resonant frequency and the anti-resonant frequency causes undesirable filter characteristics. Referring to FIG. 16, the graph shows the impedance-frequency characteristics of a conventional one-port SAW resonator when arranged on the LiTaO.sub.3 substrate. As indicated by the arrow A, the ripple occurs between the resonant frequency and the anti-resonant frequency. Further, because the conventional SAW filter combines a plurality of one-port SAW resonators to provide filter characteristics, when the ripple occurs in the one-port SAW resonators, a ripple also occurs in the filter characteristics of the SAW filter, which corresponds to the frequency of the ripples in the one-port SAW resonators. Thus, referring to FIG. 17, which shows the attenuation-frequency characteristics of the above-mentioned SAW filter, the ripple appears on the low frequency shoulder of the pass band of the SAW filter, as indicated by the arrow B. The appearance of the ripple is significant because the effect of the ripple increases the insertion loss at the low frequency side of the pass band so that the flatness of the pass band becomes even worse.
Note that according to the SAW filter of Japanese Examined Patent Publication No. 56-19765, insertion loss at the center of the pass band is less than at the shoulders of the pass band so that the flatness of the pass band needs significant improvement. When a LiTaO.sub.3 substrate is used as the piezoelectric substrate, the flatness of the pass band is even worse because of the ripples that are generated by the SAW resonators.